1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a duplexer including the same.
2. Description of the Related Art
Surface acoustic wave (SAW) filters are normally used for the RF filters and antenna duplexers of portable telephone devices.
In a mobile telecommunication system involving those portable telephone devices, the transmit band and the receive band are situated very close to each other. So as to avoid crosstalk between transmitted signals and received signals, a transmit filter is required to suppress the receive band, and a receive filter is required to suppress the transmit band. Compared with any other filter device, a SAW filter normally exhibits a sharper cut-off profile in the vicinities of the pass band. However, there is an increasing demand for SAW filters with a greater degree of suppression.
So as to increase the degree of suppression on the high frequency side of the pass band of a transmit filter or a receive filter, a capacitance element may be provided between the input terminal and the output terminal of the SAW filter. This technique is disclosed in Japanese Unexamined Patent Publication Nos. 1-135113, 11-251861, 9-172342, and 9-321569, and will be hereinafter referred to as Prior Art 1. FIG. 1B shows an electric equivalent circuit that represents the structure of Prior Art 1. For purposes of comparison, FIG. 1A shows an electric equivalent circuit of a conventional structure that does not have a capacitance element between the input terminal and the output terminal.
As shown in FIG. 1B, a SAW filter 200 of Prior Art 1 has three resonances between an input terminal 4 and an output terminal 5. In this SAW filter 200, a capacitance C is also provided between the input terminal 4 and the output terminal 5. FIG. 2 shows the results of a simulation that was carried out to examine the filter characteristics of the SAW filter 100 of FIG. 1A and the filter characteristics of the SAW filter 200 of FIG. 1B. In this simulation, the SAW filters 100 and 200 were produced as 1.9 GHz band filters.
As shown in FIG. 2, the filter characteristics of the SAW filter 100 have a relatively gentle cut-off profile on the high frequency side of the pass band. On the other hand, the filter characteristics of the SAW filter 200 have a steep cut-off profile in the vicinity of the boundary between the pass band and the stop band on the high frequency side, with a zero point appearing at approximately 2.1 GHz. The capacitance C provided between the input terminal 4 and the output terminal 5 causes the zero point in the band (a stop band) on the high frequency side of the pass band. Thus, the degree of suppression on the high frequency side can be greatly increased.
So as to increase the degree of suppression on the low frequency side of the pass band of a transmit filter or a receive filter, the input ground terminal and the output ground terminal of the SAW filter may be grounded via an inductance element. This technique is disclosed in Japanese Unexamined Patent Publication Nos. 9-162676 and 54-123892, and will be hereinafter referred to as Prior Art 2. FIG. 3 illustrates the structure of Prior Art 2.
As shown in FIG. 3, a SAW filter 300 of Prior Art 2 has interdigital transducers (IDTs) 11 between the input terminal 4 and the output terminal 5. In this SAW filter 300, the ground terminals are connected to form a common ground terminal 8, and the common ground terminal 8 is grounded via an inductance L. FIG. 4 shows the result of a simulation that was carried out to examine the filter characteristics of the SAW filter 300 of FIG. 3. In this simulation, the SAW filter 300 was produced as a 1.9 band filter. For purposes of comparison, FIG. 4 also shows the filter characteristics of the SAW filter 100 of FIG. 1A.
As shown in FIG. 4, the filter characteristics of the SAW filter 300 have a steep cut-off profile in the vicinity of the boundary between the pass band and the band on the low frequency side, with a zero point appearing at approximately 1.8 GHz. The input ground terminal and the output ground terminal of the SAW filter 300 are connected and are then grounded via the inductance L, so that the zero point can appear in the band (another stop band) on the low frequency side of the pass band. Thus, the degree of suppression on the low frequency side can be greatly increased.
However, the technique of providing a capacitance between the input terminal and the output terminal in accordance with Prior Art 1 has a drawback in that the suppression on the low frequency side of the pass band decreases, though the suppression on the high frequency side of the pass band increases. For example, the degree of suppression in the frequency band of 1.8 GHz and lower may decrease with the addition of a capacitance, as shown in FIG. 2.
On the other hand, the technique of grounding the connected ground terminals through an inductance in accordance with Prior Art 2 has a drawback in that the suppression on the high frequency side of the pass band decreases, though the suppression on the low frequency side of the pass band increases. For example, the degree of suppression in the frequency band of 2.1 GHz and higher may decrease with the addition of an inductance, as shown in FIG. 4.